The hemagglutinin (HA) is the major surface envelope protein of influenza A and B viruses. It carries essential functions in the viral life cycle. Viral entry is mediated by attachment through HA binding to sialic acid receptors on the host membrane and then internalization of viral particles into the late endosome 4. The HA receptor-binding site is a shallow depression in the globular head at the extreme membrane-distal end of HA, and is surrounded by structural elements commonly referred to as the 220 loop, 130 loop, 150 loop and 190 helix, named after their sequence numbers in the mature HA protomer 4. Although HAs of different subtypes from different hosts display some unique structural features around the receptor-binding pocket, which determine their fine specificity and avidity, a large portion of the receptor-binding site is highly conserved for the recognition of the common ligand, the terminal sialic acid of sialyated glycans. Most notably, a hydrophobic cavity at the 150 loop end of the receptor-binding site, which accommodates the 5-acetamido moiety of sialic acid, is formed by universally conserved HA residues Trp153, Leu194, Tyr195 and other conserved residues from the 130 and 150 loops.